Electroluminescent display device with piezoelectrical scanning and gating means



Dec. 6, 1966 J J. LAMBE ETAL 3,290,549

ELECTROLUMINESCENT DISPLAY DEVICE WITH PIEZOELECTRICAL SCANNING ANDGATING MEANS Filed Au 8, 1960 5 Sheets-Sheet 1 E z: 1.. VIDEO l6 l8 l2fi K ELECTRON/C DRIVER FLAT PANEL SWEEP/N6 MODULATOR 056, OR AMPL/F/ERDISPLAY 1 I l 1 22 20 30 2o 2 1NVENTOR JOHN J. LAMBE JOSEPH MUDAR SOLNUDELMAN BY 5% -r 5M ATTORNEYS Dec. 6, 1966 J. J. LAMBE ETAL 3,290,549

ELECTROLUMINESCENT ELECTRICAL DISPLAY DEVICE WITH PIEZO SCANNING ANDGATING MEANS 3 Sheets-Sheet 2 Filed Aug. 8, 1960 SWITCHING DEVICESWITCHING DEVICE INVENTORS JOHN J. LAMBE JOSEPH MUDAR SOL NUDELMAN BY5M? 8M ATTORNEYS 3,290,549 TRICAL Dec. 6, 1966 J. J. LAMBE ETALELECTROLUMINESCENT DISPLAY DEVICE WITH PIEZOELEC SCANNING AND GATINGMEANS 5 Sheets-Sheet 5 Filed Aug. 8, 1960 lOO ELECTRONIC SWEEPINGOSCILLATOR COMPARISON GATING r-IIB CIRCUIT AMPLIFIER D QQ Off.

I NVEN TOR 5 United States Patent Ofiice 3,290,549 Patented Dec. 6, 19663,290,549 ELECTROLUMINESCENT DISPLAY DEVICE WITH PIEZOELECTRICALSCANNING AND GATING MEANS John J. Lambe, Wayne, Joseph Mudar, WhitrnoreLake,

and Sol Nudelmau, Ann Arbor, Mich, assignors to Research Corporation,New York, N.Y., a corporation of New York Filed Aug. 8, 1960, Ser. No.47,989 2 Claims. (Cl. 31555) The present invention relates to displaydevices and, more particularly, to a display device employing anelectroluminescent light emitting phosphor of which selective areas maybe energized by an electric field established by one or morepiezoelectric crystal elements.

The conventional electroluminescent display devices which are capable ofeffecting energization of selected areas of the light-emitting phosphormaterial typically comprise a first grid of spaced parallel conductors,a second grid of spaced parallel conductors spaced in electricallyinsulated relationship with the first grid, the conductors of the secondgrid running at substantially a right angle to the conductors of thefirst grid, and a phosphor layer positioned between the grids. Thephosphor layer is composed of an electroluminescent phosphor emittinglight phased with the applied voltage. In operation, a switchingarrangement is employed to simultaneously energize selective ones of theconductors of the first and second grids to generate an intense electricfield in localized regions of the device where the conductors cross oneanother. The electric field causes the phosphor to luminesce as a spotand by proper synchronization of the switching arrangement, a spot canbe made to traverse the phosphor layer and thereby create a, raster. Bymodulation of the applied voltage, it is possible to vary the intensityof the emitted light and thereby display information much in the samemanner as in conventional television receiver systems.

A typical system employing such a device is illustrated and described inUS. Patent 2,698,915 entitled Phosphor Screen, William W. Piper.

The conventional crossed grid type of electroluminescent display deviceshave certain disadvantages which the present invention overcomes. Amongthese disadvantages is t-he necessity tor the use of a high-speedprecision switch with which it is difficult to obtain high resolution.Colored displays are impractical as it is necessary in order to obtaincolor displays to use a plurality of superposed layers of diiferentcolor light emitting phosphors, each layer having its own crossed gridnetwork. And, thirdly, control requirement on the phosphor Circuitry,and voltages are severe in order to avoid electroluminescence bysufficiently high electric field being developed in areas other than thecross-over points of the conductors, causing a background problem.

It is an object of the present invention to produce anelectroluminescent display device wherein discrete areas thereof may beselectively energized by varying the frequency of the energizingpotential.

A further object of the invention is to produce an electroluminescentdisplay device capable of exceedingly good resolution, definition, andbrightness characteristics.

. The invention typically may be defined as an electroluminescentdisplay devi-ce comprising a piezoelectric crystal structure shaped toprovide zones of differential response to oscillating potentials,electroluminescent means responsive to an electric field positioned as acoating on the surface of the crystal structure, and means for applyingan oscillating potential to the structure to create an electric field ata surface zone of the structure; which zone is determined by thefrequency of the applied potential.

Other objects and advantages of the invention will be manifested fromreading the following detailed description of several embodiments of theinvention when considered in light of the accompanying drawings inwhich:

FIG. 1 is a diagrammatic illustration of a system capable of employingand energizing the invention;

FIG. 2 is a diagrammatic illustration of a basic principle of theinvention wherein a plurality of discrete crystalline elements areadapted to energize the associated phosphor coatings;

FIG. 3 is a diagrammatic illustration of another embodiment of theinvention wherein a single wedge-shaped crystalline element is employedto energize selected areas of an associated phosphor coating;

FIG. 4 is a diagrammatic illustration of an alternative arrangement forenergizing an electroluminescent display device in accordance with thepresent invention;

FIG. 5 is a perspective view of a modification of the invention which iscapable of displaying conventional video television signals wherein eachscanning line of the device is similar to the arrangement shown in FIG.2 being comprised of a plurality of individual crystalline cylinders,each provided with a coating of electroluminescent phosphor material onone surface thereof;

FIG. 6 is a perspective view of another modification of the inventionwhich is capable of displaying conventional video television signalswherein each scanning line of the device is similar to the arrangementshown in FIG. 3 being comprised of a single wedge-shaped crystallineelementbeing provided with a coating of electroluminescent phosphormaterial on one surface thereof; and

FIG. 7 is a diagrammatic illustration of an embodiment of the invention'for use as an aircraft altitude meter.

The underlying principle of the present invention is that a luminescentlight spot can be created and caused to move about on a panel surface ofelectroluminescent material by varying the frequency of the drivingvoltage applied to the means tor establishing the electric field appliedto the phosphor material. The intensity of the light output will dependupon the magnitude of the driving voltage, since this voltage, in turn,is responsible for the electric field strength applied to the phosphor.As shown in the schematic diagram of FIG. 1, an electronic sweepfrequency oscillator 10 is actually all that is necessary to achieve alight spot movement on the display panel 12, while a video input 14 isfed to a modulator 16 to provide :for amplitude modulation of thedriving voltage and thereby effect an intensity modulation of theresultant luminescent spot. The output of the modulator may be fedthrough a driver amplifier network 18.

FIG. 2 illustrates one simple embodiment of a display deviceincorporating the principles of the invention. There is shown aplurality of piezoelectric cylinders 20, each being provided with anelectrically conductive coating 22 disposed on the bottom thereof. Thetop of each of the cylinders 20 is provided with an electroluminescentphosphor coating 24 and an electrically conductive coating 26. It may.be desirable in certain applications of the invention to insulatinglyseparate the coatings 24- and 26 by slight spacing therebetween oralternatively by disposing a dielectric material therebetween. It willbe noted that the phosphor coating 24 consumes the majority of the areaof the top of the cylinders 20, while the electrically conductivecoating 26 consumes only a small portion of the area of the top. Theplurality of cylinders 20 is connected in electrically parallelrelationship across a variable frequency generator 28 by electricallyconductive leads 3% and 32. The lead 30 is connected to the electricallyconductive coating 22, on the bottom of the cylinders 20, while the lead32 is connected to the electrically conductive coating 26 on the top ofthe cylinders 20. Each of the cylinders is formed of a crystallinematerial such as barium titanate and has its own resonating frequency,each differing from the other.

The phosphor layers 24 give off light through a phenomenon ofelectroluminescence which is a process by which certain semiconductingmaterials known as phosphors, emit radiant energy at room temperatureunder the primary stimulus of a varying electrical potential, whichgenerates a varying electric field. Such phosphors include, for example,gallium phosphide and zinc sulfide activated with copper. While thereare several scientific theories presently advanced to explain themechanism by which electnoluminescence occurs, a discussion of thesetheories is not essential in this description.

The material which is used to fabricate the crystalline cylinders 20 isa piezoelectric material and may be defined as one which exhibits aphenomenon of expansion along one axis and contracts on another axiswhen subjected to an applied voltage and in response thereto a strongpolarization (electric) field is generated :across its end surfaces. Theintensity of the generated field is dependent upon the applied voltage.Such piezoelectric materials may be either single crystals as quartz,Rochelle salt, or ammonium dihydrogen phosphate; or they may bepolycrystalline material such as ferroelectric barium or lead titanateceramics which have been polarized.

With the above background, it will be appreciated that when a generator28 is energized to produce a voltage of a frequency equal to theresonant frequency of one of the cylinders 20, that cylinder willrespond in its vibrational mode and a strong polarization (electric)field will be generated across its end surfaces. For purposes ofillustration, we will assume that the generator 28 is producing avoltage of a frequency equal to the resonant frequency of the centermostcylinder 20 of the plurality illustrated in FIG. 2. The intensity of thefield produced or developed by the resonating cylinder is dependent uponthe voltage output of the generator 28. The electric field establishedacross the end surface of the resonating cylinder is sufficient toenergize the electroluminescent coating 24 and thereby cause light to beemitted therefrom. It will be appreciated that by changing the frequencyof the generator 28, the different cylinders 20 can be made to respondselectively, with the light output following the tuned cylinder 20 andthe light modulation being achieved by modulating the amplitude of thevoltage output of the. generator 28.

Similar results, that is, the ability to effect selective areaenergization of an electroluminescent coating, may likewise be achievedby the structures shown in FIG. 3. In the structure shown in FIG. 3, acrystalline wedge 20' similar to the cylinders 20 of FIG. 2, made ofquartz or the like material, and is provided with an electricallyconductive coating 22' on the bottom surface thereof. Anelectroluminescent phosphor layer 24 is coated on the major portion ofthe top of the wedge 26) and an electrically conductive coating 26 onthe remaining portion of the top thereof. The wedge is electricallyconnected to frequency generator 28 through conductors 30 and 32.Localized resonance of the wedge 20' is obtained by varying thefrequency of the generator 28' which will accordingly enengize selectiveareas of the electroluminescent phosphor coating 24'. Since the resonantfrequency of the crystalline material of the wedge 30 is a sharpfunction of the geometry of the wedge, localized resonance may bereadily attained.

It will be appreciated that of primary importance in the application ofdevice of the type already explained is that the movement of the lightspot or energized area of the phosphor coating may be generated withoutthe need for electrodes other than the conductive coatings on the topand bottom of piezoelectric material to be placed in close proximity tothe display device. The electric field which is effective to energizethe phosphor material is generated by polarization charges appearing atthe surface of the crystal. These changes can be made to appear atsurfaces to which no electrical connections are made. If the crystallineelements are properly shaped, the vibrational polarization fields can beinducedwith the electrodes placed a significant distance along thecrystal away from the surface thereof which is coated with the phosphorlayer.

It has been found advantageous in certain applications where it isdesirable to obtain a maximum field across the surface of the energizingcrystal to employ a structure such as shown in FIG. 4. In thearrangement illustrated in FIG. 4, there is a crystalline element 40provided with an electroluminescent phosphor coating 42 on the uppersurface thereof. A transparent electrically conducting coating 43, suchas stannic chloride on a sheet of glass 44, is disposed over the entireuppermost surface of the phosphor coating 42. The base of the element 40is provided with an electrically conductive coating 46 which may incertain applications be transparent to thereby allow the light emittedfrom the phosphor coating 42 to be viewed from both sides, thecrystalline material of the element 40 must either be transparent ortranslucent. Both the electrically conductive coatings, 43 and 46 arecoupled together through an electrically conducting wire 48. A generator50, capable of producing a variable frequency voltage, has one of itsoutput terminals connected to an electrically conductive strip 54through an electrically conducting wire 52. The strip 54 is adapted tobe in intimate contact with the side walls of the crystalline element40. The other output terminal of the generator 50 is connected to theelectrically conductive coating 46 through an electrically conductingwire 56.

In operation, the generator 50 is effective, when operated at theresonant frequency of the crystalline element 40, to create :an electricfield across the surface thereof. The conducting coatings 43 and 46provide a minimum path length for the electric field and, therefore,achieves maximum therein. This action is similar and can be compared tomoving the plates of a parallel plate condenser closer to one another,while maintaining the same potention difference across the plates. Sincethe field intensity is given by the following relation:

where d is the distance between the plates and v is the voltage acnossthe plates. Therefore, by maintaining a given applied voltage, the fieldintensity E may be increased by making the distance d between the platessmaller. It is also to be understood that the arrang ment illustrated inFIG. 4 has an advantage in that the voltage applied directly to theelectrode strip 54 also contributes a field across theelectroluminescent phosphor layer 42 in addition to the polarizationfield created by the piezoelectric material of the crystal 40.

This device also provides significant voltage amplification. Thepotential difference across the phosphor has been found to be three tofour times the voltage driving the piezoelectric element (at resonanceoperation). Using a driving voltage of about v. light output comparableto the TV type displays has been obtained.

In an application where the device is employed as a TV type display, forexample, arrangements such as illustrated in FIGS. 5 and 6 may be used.In FIG. 5 an arrangement of piezoelectric cylinders 60 are disposed in apredetermined pattern on a layer 62 of an electroluminescent phosphorwhich, in turn, is desposited on a transparent supporting panel 66formed of glass or the like material. The supporting panel 66 isprovided with the transparent conductive coating 68. The opposite end ofthe cylinders 60 are coated with an electrically conductive material 64.

The various groups of cylinders 60 are connected together in rows inparallel by Wires 69 and 70 to a switching device 72 which, in turn, isconnected to a generator 74 capable of producing variable frequencysignals. It will be noted that the cylinders 60 of each row vary inlength from one side to another in an arrangement similar to thatillustrated in FIGURE 22, therefore, each cylinder will resonate andestablish an energizing field for the phosphor layer at a differentfrequency. When the generator 74 is tuned to a frequency at which one ofthe cylinders 60 will resonate, that cylinder will create an electricfield at the end thereof in contact with the electroluminescent layer 62and cause a localized zone thereof to fluoresce. The intensity of thefield created by the individual cylinders 60 is a function of thevoltage output of the generator 74; the higher the voltage, the higherthe intensity of the field and the resultant emitted light.

By varying the generator frequency, the various cylinders 60 of a singlerow may be made to respond one at a time, with the light outputfollowing the tuned cylinder, and the light modulated in accordance withthe modulation of the generator voltage. At the completion of theenergization of .an entire row of cylinders 60, the switching circuit 72may be caused to switch the voltage signal produced by the generator 74,to selectively energize adjacent rows of cylinders. It will beappreciated by those skilled in the art that by proper synchronizationthe cylinders 60 can be energized in such a manner as to produce araster much in the same way a raster is produced in the conventionalcathode ray tubes used in television reception.

Another embodiment of the invention is illustrated in FIG. 6 wherein anarrangement of piezoelectric wedges 80, each being of the typeillustrated in FIG. 3, is disposed on a layer 82 of anelectroluminescent phosphor which, in turn, is deposited on atransparent electrically conductive coating 84, of a supporting panel86. The opposite side of the wedges 80 from that in contact with thephosphor is provided with an electrically conductive coating 88. Eachwedge 80 is connected to a variable frequency generator 90 through aswitching circuit 92 by an electrically conducting wire 89. The wires 89may be cabled together as shown. The conductive coating 84 is likewiseconnected to the switching device 92 through an electrically conductingwire 91. The generator 90 is capable of producing variable frequencysignals to cause various zones of the wedges 80 to resonate and therebycreate associated electric fields to excite the electroluminescentphosphor 82. Thereby a light spot can be produced and caused to movealong a wedge 80 by merely varying the frequency of the generator 90.Then, as in the embodiment illustrated in FIG. 5, the adjacent wedges 80may be selectively energized by the switching circuit 92. The resolutionavailable with a wedge is determined mainly by the piezoelectricmaterial from which the wedges are made and geometry of the wedge. Forall intents and purposes, the embodiment illustrated in FIG. 6 isoperated in the same manner as the embodiment of FIG. 5. Accordingly, ifthe generator 90 is operating to produce a voltage signal of a givenfrequency, only one spot on the energized wedge will establish a fieldof the proper intensity to cause light to be emitted from a single zoneof the phosphor layer 82. A change in the frequency of the appliedvoltage causes the light spot to move.

If bandwidth requirements permit, all of the wedges 80 can be placedelectrically in parallel and by a single traversal through the frequencyrange, the spot would move over the entire panel.

With limited bandwidth requirements, the wedges 80 are connected asshown in FIG. 6 and are, in operation, frequency scanned one at a timein any desired sequence 'by the switching device 92 and will displayvideo information by amplitude modulation of the electric fieldgenerator 90.

With reference to FIG. 7, there is shown a system embodying theprinciples of the invention in an aircraft attitude meter. The systememploys a single wedge-shaped crystalline element similar to thatillustrated in FIG. 3. The wedge element 100 is formed of quartz orother suitable material which is substantially transparent to the lightemitted from an associated electroluminescent phosphor material layer102 coated on one side of the element. A conductive coating 104 isapplied to the outer surface of the phosphor layer 102 and may be alight transparent material such as stannic chloride in which case thelight emitted from the phosphor coating may be viewed from two sides ofthe wedge element 100, or may be formed of an opaque conducting materialwhich would obviously permit viewing of the light emitted from thephosphor layer 102 only through the material of the element 100. Theconductive layer 104 may cover the entire exposed surface of thephosphor layer 102 as illustrated in FIG. 4 or may be applied in themanner illustrated in FIGS. 2 and 3. The conductive layer 104 is shownas being connected to ground potential through a conductive wire 106.

A strip 108 of electrically conductive material, similar to the strip 54of FIGURE 4, is applied to the side walls of the element 100 and is, inturn, connected to the energizing circuit through a conductive wire 110.

The surface of the element 100 which is opposed to the surface on whichthe phosphor layer 102 is applied is provided with indicia indicatingvarious altitudes. If wedge 100 is formed of non-transparentpiezoelectric material, the phosphor layer should be positioned betweenthe wedge and an indicia bearing member.

The energizing circuit comprises an altitude sensing device 112 which iscapable of producing a DC. output signal proportional in strength to thesensed altitude. Conventional radar systems may be satisfactorilyemployed for this purpose. The output signal produced by the altitudesensing device 112 is fed to a comparisongating circuit 114. Thecomparison-gating circuit 114 is capable of producing a saw-toothreference signal. The comparator portion of the circuit 114 compares theDC. signal received from the sensing device 112 with the saw toothreference signal. The slope of the saw tooth signal is set to correspondto the altituderange required for the sensing device 112. The DC signalfrom the sensing device 112 will then exceed the saw tooth signal up toan altitude match. The saw tooth reference signal of thecomparison-gating circuit is synchronized with a sweep frequency signalfed thereto by a sweep frequency generator 115. Thereby, the saw toothreference signal commences when the sweep frequency generator 116 startsits frequency scan. For all levels of the DC. signal from the sensingdevice 112 which exceed the reference signal, a gate is opened and theoutput of the sweep frequency generator 116 is passed to an amplifier118 and to the altitude display device through the conductor 110.

In operation, therefore, when the altitude sensing device 112 senses asignal corresponding to 11,000 feet, for example, a DC. signal is fed tothe comparison-gating circuit 114 which is operative to open a gate andallow only that frequency signal of the generator 116 effective toenergize the top portion of the wedge element 100 to pass to theamplifier 118 and thence to the conductive strip 108 of the wedgeelement 100 through the conductor 110. Whereupon the electroluminescentphosphor material 102 adjacent the upper portion of the wedge element100 is excited to give off visible light indicating an altitude of11,000 feet.

As in the embodiment of FIGURE 4 the altitude meter piezoelectriccrystal display element 100 provides significant voltage amplification.Thus, at any resonant frequency applied to the crystal wedge 100 throughconduc tor 110, the potential difference across the phosphor 102 will befound to exceed the driving voltage applied between str-ip 108 andground return wire 106.

Other satisfactory systems employing the concepts of the inventioninclude the use of a sweep frequency oscillator with a voltage tuneablebandwidth. Thus, as the voltage increases with altitude, so will thebandwidth of the sweep frequency oscillator and the display will lightup accordingly.

Although the description of the devices has made reference to theelectroluminescent material as being phosphor, it must be understoodthat satisfactory results could be obtained by employing other materialswhich are electroluminescent. These materials include electric fieldsensitive fluorescent liquids and jells, or gases with suitabledischarge characteristics. In use, these materials are contained in athin panel, one surface of which is in contact with the piezoelectricmaterial.

Display devices such as those hereinabove described and illustrated inthe drawings also may be employed as frequency spectrum analyzers. Insuch an application, the signal to be examined is fed into a variable,sharply tuned, resonant circuit synchronized to the sweepingoscillation. The tuned circuit employs a voltage dependent capacitor tovary the resonant frequency of the circuit. The output of the tunedcircuit is rectified and filtered to eliminate the carrier. This videosignal is then fed to the modulator 16 of FIG. 1 and thence to thedisplay device 12 through an amplifier 18 and thereby cause the displaypanel 12 to exhibit visibly the frequency characteristics of theincoming signal.

In accordance with the provisions of the patent statutes, we haveexplained the principle and mode of operation of our invention and haveillustrated and described what we now consider to represent its bestembodiments. However, we desire to have it understood that, within thescope of the appended claims, the invention may be practiced otherwisethan as specifically illustrated and described.

We claim:

1. An electroluminescent display device comprising a Wedge shapedpiezoelectric crystal element, an electroluminescent material disposedon a tapering longitudinal surface of said crystal element, an electrodedisposed on the same surface as said material, said electrode and saidmaterial extending along the entire length of said surface, thewidthwise dimension of said electrode, with respect to the width of saidtapering longitudinal surface, being less than the widthwise dimensionof said electroluminescent material, and a base electrode extending overthat surface of said crystal which is opposite to said tapering surface.

2. The display device of claim 1 wherein said electrode on said taperingsurface is positioned along one longitudinal edge thereof.

References Cited by the Examiner UNITED STATES PATENTS 2,159,891 5/1939Guerbilsky 32456 X 2,779,191 1/1957 Willard 3l08.1 X 2,796,584 6/1957Hurvitz 313108 2,810,883 10/1957 Carnine 32478 2,877,371 3/1959 Orthuber315-169 2,951,168 8/1960 Yando 3l5-55 X 3,235,799 2/1966 Hurvitz 324-78JAMES W. LAWRENCE, Primary Examiner.

RALPH G. NILSON, ARTHUR GAUSS,

GEORGE N. WESTBY, Examiners.

o C. R. CAMPBELL, Assistant Examiner.

1. AN ELECTROLUMINESCENT DISPLAY DEVICE COMPRISING A WEDGE SHAPEDPIEZOELECTRIC CRYSTAL ELEMENT, AN ELECTROLUMINESCENT MATERIAL DISPOSEDON A TAPERING LONGITUDINAL SURFACE OF SAID CRYSTAL ELEMENT, AN ELECTRODEDISPOSED ON THE SAME SURFACE AS SAID MATERIAL, SAID ELECTRODE AND SAIDMATERIAL EXTENDING ALONG THE ENTIRE LENGTH OF SAID SURFACE, THEWIDTHWISE DIMENSION OF SAID ELECTRODE, WITH RESPECT TO THE WIDTH OF SAIDTAPERING LONGITUDINAL SURFACE, BEING LESS THAN THE WIDTHWISE DIMENSIONOF SAID ELECTROLUMINESCENT MATERIAL, AND A BASE ELECTRODE EXTENDING OVERTHAT SURFACE OF SAID CRYSTAL WHICH IS OPPOSITE TO SAID TAPERING SURFACE.